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C: Energy Conversion and Storage; Energy and Charge Transport

Evidence for Conduction Band Mediated Two-Electron Reduction of a TiO-bound Catalyst Triggered by Visible Light Excitation of Co-adsorbed Organic Dyes 2

Valeria Saavedra Becerril, Elin Sundin, and Maria Abrahamsson J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07669 • Publication Date (Web): 24 Oct 2018 Downloaded from http://pubs.acs.org on November 2, 2018

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The Journal of Physical Chemistry

Evidence for Conduction Band Mediated TwoElectron Reduction of a TiO2-bound Catalyst Triggered by Visible Light Excitation of Coadsorbed Organic Dyes. † † Valeria Saavedra Becerril, Elin Sundin , Maria Abrahamsson*.

Department of Chemistry and Chemical Engineering. Chalmers University of Technology, 41296 Göteborg, Sweden. AUTHOR INFORMATION † These authors contributed equally * [email protected]

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ABSTRACT. Here we report direct spectroscopic evidence of a visible light-induced twoelectron transfer reaction to a molecular model catalyst, facilitated by conduction band mediation. Using a simple, yet remarkably reproducible, co-sensitization approach we anchored dyes and catalysts to mesoporous TiO2 thin films, at dye:catalyst ratios of 30:1, 15:1 and 8:1. The distinct optical spectroscopic signatures of different redox states of the catalyst used, Coprotoporphyrin IX, allows for transient absorption detection of intermediate and final photoproducts in the photoexcited co-sensitized dye-TiO2-catalyst assemblies. We show that by tuning the dye:catalyst ratios, control over whether photoexcitation results in one or two electrons being transferred to the catalyst is achieved. The two-electron transfer process is favored by a large excess of photosensitizer compared to catalyst.


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Introduction The conversion of solar energy into fuels has potential to provide society with clean and storable energy. Fuel forming reactions rely on multiple electron transfer reactions, thus multiple charges or redox equivalents must be generated and stored, while matching the turnover frequencies of the reactions.1-4 Dye-sensitized mesoporous semiconductors are promising materials to perform such reactions due to their ability to store many charges5-6, sustain charge separation on millisecond timescales7-8 and transfer the charges to anchored molecules.9-11 By co-attaching molecular dyes and catalysts, the conduction band (CB) can serve as a mediator for electron transfer events between them. Such dye-sensitized photocatalysis (DSP) assemblies have been used for H2 generation as well as reduction of CO2. However, DSPs typically show low efficiencies.12-17 This has been proposed to be due to difficulties in controlling the electron transfer steps14, 17-18 which highlights the need for detailed understanding of the factors governing the electron transfer processes. Furthermore, a recent publication by Gatty et al shows that seemingly similar dye-sensitized systems may behave very differently, also emphasizing the need for detailed mechanistic knowledge. 19 Durrant and co-workers have studied two-electron transfer reactions from TiO2 to anchored catalyst molecules by measuring the electron lifetime in the conduction band, reporting that the second reduction of the catalyst was at least 4 orders of magnitude slower than the first. They concluded that all catalyst molecules undergo the first reduction before any get doubly reduced14 and proposed a stepwise mechanism, but no intermediates were identified. Here, we outline an approach that allows for both direct spectroscopic identification of the different intermediates and that can provide kinetic information, by investigation of the electron transfer across the CB at µs-ms time-scales. In this study we are using co-sensitized TiO2 thin films 3 ACS Paragon Plus Environment


with the organic dye D35 as the photosensitizer20 and cobalt protoporphyrin IX (CoPPIX) as the model catalyst and electron acceptor. The metal center in this porphyrin (Co3+) can be reduced twice, at electrochemical potentials matching the energetic requirements for conduction-band mediation, see Scheme 1. Combining D35 and CoPPIX allows for almost exclusive excitation of the dye, and the different redox states of CoPPIX can be distinguished by analysis of the difference spectra, accounting for spectral features from all relevant species, vide infra. The concentration of electrons in the CB was kept constant to directly monitor the effect of the surface concentration of CoPPIX on the electron transfer processes.

Scheme 1. Representation of the relevant energies and redox potentials of the studied system showing that CBM is a thermodynamically allowed process. D35 redox potentials are as reported by Ellis et al. 20 All values are referenced vs NHE.

Methods Thin-film preparation. Semi-transparent TiO2 thin films (~6 µm thick) were prepared by doctor-blading deposition of commercially available nanoparticle paste (Dyesol 18NR-T, average particle diameter 25 nm), on FTO-coated glass (Pilkington, TEC-15) substrates. Glass substrates were cleaned by ultrasonication in a solution of 2% RBS detergent followed by 4 ACS Paragon Plus Environment

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ethanol and/or acetone prior to paste deposition. After paste deposition, the samples were progressively heated to 450 ºC in air flow for 30 minutes. Dye sensitization. To prepare co-sensitized films, solutions of different D35:CoPPIX molar ratios were prepared by adding small amounts of a 1.0 mM CoPPIX solution in DMSO to a solution of 0.3 mM D35 in ethanol. We kept the number of D35 molecules at the surface constant for the different D35:CoPPIX ratios to allow for investigation of how the formation of the different reduced states of CoPPIX is affected by the D35:CoPPIX ratios at the TiO2 surface. Thus, the number of injected electrons is the same in all films used, as long as the excitation intensity is kept constant. By doing so, we avoided effects from varying density of injected electrons. For the preparation of co-sensitized films, they were heated to 120º C to remove all water prior to immersion in the solutions. The samples were kept in the solutions for 15 minutes and then rinsed with ethanol. The different solutions and the corresponding ratios at the TiO2 surface are shown in Table 1 below

Table 1. D35:CoPPIX molar ratios in the sensitizing solutions and the obtained molar ratios at the TiO2 surface after 15 minutes of sensitization Solution molar ratios

Resulting molar ratios at the TiO2 surface

2:1

8:1

5:1

15:1

10:1

30:1

Spectroscopic characterization. Electronic absorption spectra were recorded using a Varian Cary 50 Bio spectrophotometer. In the photolysis experiments, absorption spectra were recorded while a Xe-arc lamp was used for continuous irradiation. The light from the Xe-arc 5 ACS Paragon Plus Environment


lamp was filtered through a band-pass filter (250-350 nm), as well as an IR filter to avoid heating. Spectra were collected every 30 s. Single-wavelength transient spectroscopic measurements were performed using a Continuum Surelight II Nd:YAG laser at 10 Hz repetition rate with a Continuum Surelight optical parametric oscillator, producing pulsed (ca 8 ns fwhm) excitation light. The excitation wavelength was set to 480 nm to achieve selective photoexcitation of the dye D35 with minimum simultaneous photoexcitation of the CoPPIX catalyst. The irradiation intensity was kept constant at 0.2 mJ cm-2. The probe light was generated by a radiometric power supply (Newport) powering a quartz tungsten halogen lamp (Alfalux/Ushio). The probe light wavelength was selected using two Cornestone 130 monochromators (Oriel Instruments). Data was collected using a Costronics photodetector amplified by a Costronics Optical Transient Amplifier connected to a Tektronix TDS 2022 oscilloscope. Electrochemical characterization. Cyclic voltammetry and bulk electrolysis of CoPPIX was performed using a CH650-A electrochemical workstation, CH instruments, in a threeelectrode cell configuration with a glassy-carbon disk working electrode, SCE reference electrode and Pt-disk counter electrode. Ferrocenium/ferrocene (Fc+/0) redox couple was used as an internal standard. Electronic absorption spectra were recorded using a Varian Cary 50 Bio spectrophotometer for spectroelectrochemical characterization.

Results Electronic spectroscopy and electrochemical characterization. The UV-Vis spectra of D35/TiO2 and CoPPIX/TiO2 are shown in Figure 1a. The observed spectral features are in agreement with literature reports.21-22 TiO2 films with varying dye:catalyst ratios were prepared and their corresponding electronic absorption spectra are shown in Figure 1b. To allow meaningful comparisons, the concentration of dye was kept constant while the amount of 6 ACS Paragon Plus Environment

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CoPPIX was varied to yield different ratios. These were determined by assuming that the molar absorptivity of the molecules on the film are the same as in solution and by neglecting the minor contribution from CoPPIX to the absorbance at 500 nm (see Equation S3 in the supporting information). The surface coverage of the films was calculated to ∼ 370 D35 molecules per nanoparticle, which translates to 12, 25 and 46 CoPPIX molecules per nanoparticle for the 30:1, 15:1 and 8:1 ratios respectively. The calculation is outlined in the supporting information (Eq. S1-S3). Cyclic voltammetry of CoPPIX in DMSO revealed one reversible peak at -1.4 V and one quasi-reversible peak at -0.1 V vs Fc0/+ which can be reasonably assigned to the Co(II/I) process and Co(III/II) respectively (Figure S1).23 Spectroelectrochemistry was, for practical reasons, performed in DMSO, Figure 2a, and revealed that a one-electron reduction is accompanied by an absorption decrease at 425 nm and an increase around 405 nm and the Q-bands merge into a less structured but slightly more intense feature at 560 nm. A decreased absorption at 405 nm and a regrowth of the band around 425 nm with a slight blue-shift is observed for the doubly reduced species. The absorption of the Q-band decreases and separates into distinct peaks centered at 536 and 568 nm. These changes agree with previous observations for similar Coporphyrins attached to TiO2. 11

Figure 1. Electronic absorption spectra of a) D35/TiO2 and CoPPIX/TiO2 and b) D35CoPPIX/TiO2 films with different dye:catalyst ratios at the surface. 7 ACS Paragon Plus Environment


Figure 2. a) Absorption spectra of CoPPIX in 0.1 M TBAPF6/DMSO in its initial state (Co3+) and after bulk electrolysis at the potentials indicated. b) Absorption changes of CoPPIX /TiO2 in CH3CN, during direct band gap excitation (λ = 250-350 nm).

Following the approach of Meyer11, conduction band mediated (CBM) two-electron transfer to CoPPIX was proven through band gap excitation of the TiO2 by UV irradiation (λexc=250350 nm) and simultaneous monitoring of the corresponding spectral changes, Figure 2b. This experiment was performed in CH3CN, and from comparison with spectroelectrochemistry (Figure 2a), formation of both [CoPPIX]- and [CoPPIX]2- are confirmed, however one must note that in CH3CN, the spectra are redshifted ~5 nm compared to DMSO. As can be seen in these figures, [CoPPIX]- and [CoPPIX]2- has distinctly different spectral features from 530 to 580 nm, and it is therefore possible to distinguish between these products by monitoring in that region. It should be noted that the spectral differences between the CoPPIX resting state and the doubly reduced species are small and therefore require an in-depth analysis to allow for identification of the doubly reduced species.

Transient absorption studies. Proof that visible light excitation can result in a conduction band mediated electron transfer process was achieved using nanosecond transient absorption (TA). Kinetic information about the reduction of CoPPIX on µs-ms time scales was obtained 8 ACS Paragon Plus Environment

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by comparing the TA decays of TiO2 substrates sensitized with D35 only with D35/TiO2/CoPPIX assemblies with dye:catalyst molar ratios of 8:1, 15:1 and 30:1 respectively in CH3CN. An excitation wavelength of 480 nm was chosen to mainly excite D35. Control experiments show that excitation of CoPPIX/TiO2 at 480 nm does not produced any appreciable transient absorption signal. The probe wavelength was varied depending on which process that was monitored, as detailed below.

Figure 3 shows the differential spectra of oxidized D35 as well as the differential spectra of [CoPPIX]- and [CoPPIX]2-. The charge recombination to regenerate the D35 ground state was monitored at 680 nm. Monitoring at this wavelength is justified by the strong positive absorption feature of [D35]+ in this region, taken together with the fact that both reduction products of CoPPIX are spectroscopically silent at 680 nm. To monitor the CBM reduction of CoPPIX, we chose three different probe wavelengths; 438 nm, 550 nm and 555 nm. Probing at three wavelengths is necessary to be able to distinguish a one-electron reduction from a twoelectron reduction, due to the similarities of the spectral features of the CoPPIX resting state and the doubly reduced state. The signal from [D35]+ is close to zero at 438 nm and slightly negative or positive at 550 nm and 555 nm respectively. The small contribution from [D35]+ at these wavelengths allows for almost exclusive monitoring of the different redox states of CoPPIX, which is not possible at other wavelengths (see Figure 3). At 438 nm, a negative feature is expected for singly reduced CoPPIX while the doubly reduced species is expected to have similar absorption as the ground state taking into account the ~5 nm red shift in CH3CN compared to DMSO. However, at 550 nm and 555 nm, the formation of [CoPPIX]- should yield a positive signal while the formation of [CoPPIX]2- would result in a negative signal. Since the contribution from [D35]+ differs at 550 nm and 555 nm, it should be possible to distinguish between the reduction products by probing at both of these wavelengths. 9 ACS Paragon Plus Environment


Figure 3. Left: Transient absorption spectrum of a D35/TiO2 thin film in CH3CN obtained at 10 μs after 500 nm pulsed excitation. Right: Spectral absorption difference spectra between CoPPIX and its two reduced states, obtained in DMSO by spectroelectrochemistry.

The transient absorption decay data is presented in Figures 4 and 5. Visual inspection of Figure 4, suggests that co-sensitization with D35 and CoPPIX slows down the charge recombination to the oxidized D35, and that this becomes especially apparent after ca 100 µs. This is also confirmed by comparing the commonly used24-27 t1/2 values for the recombination, Table 2. However, as is evident from Figure 4, the t1/2 values do not contain all the relevant information; for example, the decays of 8:1 and 15:1 at long times fall off markedly faster than the 30:1 decay.


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Figure 4. Single wavelength TA kinetics of TiO2 films with different ratios of D35 and CoPPIX after pulsed 480 nm laser excitation in CH3CN, t1/2 is shown as a black dotted line. ∆A recorded at 680 nm.

Table 2. Average half-life (t1/2) of recombination from the CB to oxidized D35 for different ratios of D35 and CoPPIX for triplicate samples. Ratio D35:CoPPIX t1/2 (ms)a

a

1:0

0.27 ± 0.04

30:1

0.55 ± 0.04

15:1

1.10 ± 0.41

8:1

2.40 ± 0.12

The indicated uncertainties represent the standard deviation obtained from analysis of triplicate

samples.

TA-decays at 438, 550 and 555 nm are shown in Figure 5. At 438 nm, ratios 8:1 and 15:1 display an apparent biphasic growth (on a log-log plot) of the TA-signal, while for the 30:1 samples, a continuous growth is observed, which does not decay completely back to zero within the time frame studied, suggesting a quantitative difference between the 30:1 and the other 11 ACS Paragon Plus Environment


D35:CoPPIX ratios. Traces recorded at 550 nm (Figure 5 d-f), reveal the expected positive feature and similar spectral features at early times for all ratios, where the positive ∆A signal is consistent with the formation of [CoPPIX]-. In contrast, D35/TiO2 samples exhibited the expected negative signal corresponding to the ground state bleach of the dye (Figure S2). Thus, the positive feature observed in co-sensitized films can be reasonably assigned to the oneelectron reduced catalyst, [CoPPIX]-, demonstrating the first visible light induced CBM electron transfer process. Visual inspection of the decay traces reveals that the fastest decay occurs in samples with the largest excess of dye (Figure 5d-i), and interestingly, the signal at 550 nm becomes slightly negative after ∼ 3 ms for the 15:1 ratio, while the 30:1 ratio display an almost constant negative feature at long times starting at ~1 ms (Figure 5e-f). As shown by spectroelectrochemistry (Figure 2a), this is expected when [CoPPIX]- is further reduced to [CoPPIX]2-, however [D35]+ would also give rise to a negative signal at 550 nm, and thus, the origin of the negative signal is ambiguous. However, probing at 555 nm will provide the desired information needed to determine the origin of the sign change, Figure 5g-i.


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Figure 5. Single wavelength transient absorption kinetics of TiO2 films with different ratios of D35 and CoPPIX after pulsed 480 nm laser excitation in CH3CN. ΔA recorded at 438 nm (ac), 550 nm (d-f) and 555 nm (g-i).

At 555 nm, [D35]+ exhibits a small positive signal (Figure 3 and Figure S2), and therefore, a negative ∆A signal should not originate from the oxidized dye. The 8:1 and 15:1 ratios again exhibit similar decays, with similarly shaped log-log decays. The lowest ratio reaches the baseline at ~7 ms, and the intermediate ratio displays a small negative signal at very long times. The decays for 30:1 samples are again different, suggesting a different process is dominating at long times. Firstly, a plateau is reached around 10-5 s, followed by a decrease in amplitude at 10-4 s, and eventually the signal becomes negative just after 0.3 ms and again reaches a stable value. Moreover, the initial amplitude for the 30:1 sample is decreased compared to the 8:1 and 13 ACS Paragon Plus Environment


15:1 species, which suggests a smaller concentration of the singly reduced species is present. Spectroelectrochemistry suggests that the change in sign should be more apparent at 555 nm compared to 550 nm, as the absorbance of [CoPPIX]2- is more negative at this wavelength relative to the initial state, see Figure 3. Control experiments confirm that reduction of CoPPIX cannot be obtained through direct excitation but is originating from photoexcitation of D35 (Figures S3-S5). Discussion Evidence for conduction band mediated two electron transfer. Firstly, we consider the lifetime of the charge-separated state induced after electron injection from the photosensitizer. Longer-lived charge separation is observed in all co-sensitized films compared to D35/TiO2 samples. Given that the dye surface coverage was kept constant, it is evident that co-attaching catalysts slows down the charge recombination in the assemblies. Secondly, all co-sensitized samples display spectral features at 550 nm that can only be explained by a reduction of the CoPPIX catalyst. Furthermore, the spectral signatures of [CoPPIX]- appear before our time resolution (< 1 µs), indicating that electron transfer to CoPPIX is favored compared to interfacial back electron transfer to the oxidized dye. Whether a second reduction step will occur depends on the number of available electrons in the CB. For the particular surface coverage and excitation intensity used in our transient absorption experiments, we calculated that there are ~ 15 injected electrons per nanoparticle (Eq. S4). Thus, the number of electrons available per CoPPIX molecule increases for the different ratios in the order 8:1 < 15:1 < 30:1, such that there are ~1.25 electrons available in the conduction band per CoPPIX molecule at 30:1, whereas for ratios 15:1 and 8:1 there are less than one electron per CoPPIX molecule. Thus, one would expect single electron transfer events to yield singly reduced CoPPIX for 8:1 and 15:1, while in the 30:1 case, one could expect 14 ACS Paragon Plus Environment

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both singly and doubly reduced CoPPIX species as the photoproducts. This is also consistent with the observed differences in the transient absorption data. Unsurprisingly, the observations suggest that if more than one electron is available in the CB per CoPPIX molecule, the probability for two electrons to find the same catalyst twice before either recombining to the oxidized dye or being transferred to another, yet not reduced, CoPPIX molecule increases. The fact that both [CoPPIX]- and [CoPPIX]2- species are subsequently observed within our time resolution, implies that formation of the [CoPPIX]- intermediate is indeed necessary, as suggested by Reynal et al.14 This interpretation is also consistent with our band gap excitation data and previous studies by e.g. Meyer and by the recent work by Chen et. al.28 It is also in good agreement with catalyst loading-dependent activities observed for DSP systems17 as well as with the study of the electron lifetime in the CB in a co-sensitized system14. Furthermore, when there is less than one electron available in the CB per CoPPIX molecule, we qualitatively observe a higher initial amplitude of the signals corresponding to singly reduced CoPPIX, suggesting that at this condition the formation of singly reduced CoPPIX occurs more efficiently. This analysis is valid when comparing a well-defined subset of data, but it is important to note that the molar absorptivity differ between the different redox states of CoPPIX and for a quantitative analysis, this must be taken into account. Based on the above discussion, we propose a mechanism for reduction of [CoPPIX]0 to obtain [CoPPIX]- and [CoPPIX]2- through a stepwise CBM electron transfer originating from the visible light photoexcitation of the dye molecules bound to TiO2. The detailed steps of this mechanism are described in Scheme 2.



Scheme 2. Mechanism for the reduction of CoPPIX by CBM stepwise electron transfer, activated by visible light photoexcitation of D35. The proposed mechanism is based on the following: 1) Electron recombination to the dye radical cation is slowed-down in presence of the catalyst. This implies that the electrons need to overcome an energetic barrier, as is expected for recombination through the conduction band. This is especially apparent for the lowest dye:catalyst ratio, suggesting that the first electron transfer is favored with increasing concentration of CoPPIX at the TiO2 surface. 2) Formation of [CoPPIX]2- occurs more efficiently for the largest dye:catalyst ratio, which we attribute to an increased probability for the electrons in the CB to find the same catalyst molecule twice. 3) A sequential electron transfer is supported by the change in sign of the transient absorption signal at 555 nm, Figure 5. This is explained by the transformation of [CoPPIX]- to [CoPPIX]2and agrees with our spectroelectrochemical and ns-transient absorption measurements. 4) Direct electron transfer from dye to catalyst as well as auto-reduction of the catalyst can be precluded based on control experiments with a co-sensitized ZrO2 film and solution studies of D35 and CoPPIX where no electron transfer between them are observed (Figures S4 and S5), supporting the proposed mechanism. 16 ACS Paragon Plus Environment

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Kinetics of the electron transfer processes. As noted above, the commonly used t1/2 values provide an easy comparison between studies, but do not contain all the relevant information for a detailed kinetic analysis of the system. It is not straightforward to develop a kinetic model for the assemblies at hand. Exponential decays as well as stretched exponential models were tested, however with unsatisfactory results. A complete kinetic analysis is beyond the scope of this paper and would also require access to better time resolution data, such that the formation of singly reduced CoPPIX could be quantified. Suffice to say that here, we observe formation of the doubly reduced species within 2 ms at the 30:1 dye:catalyst ratio. This should be contrasted to the previously reported > 40 ms to obtain a doubly reduced catalyst in a co-sensitized TiO2 sample.14 Moreover, the formation of our second reduction product, achieved without the use of a hole scavenger, from the TiO2-CB to yield [CoPPIX]2- is complete within 2 ms and then remains stable.

Conclusions In summary, we have shown that two-electron transfer from an organic dye to a cobalt protoporphyrin can be obtained in co-sensitized TiO2 samples by visible-light photoexcitation of the dye. One-electron reduction of the catalyst occurs within the time-resolution of our instrument (ca 1 µs) and the doubly reduced species can be generated in less than 1 ms, if the ratio of dye to catalyst is favorable. Furthermore, the charge separation of the doubly reduced state is sustained over milliseconds even without a sacrificial donor. Our approach makes it possible to identify the two different reduced states of the model catalyst and thus this work represents an important step towards mechanistic understanding of visible light induced heterogeneous reductive photocatalysis. Studies targeting a more detailed understanding of the kinetics as well as how the quantum yield of the doubly reduced species can be governed are underway in our laboratory. 17 ACS Paragon Plus Environment


ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Cyclic voltammetry of CoPPIX, calculation of dye surface coverage, dye-catalyst surface ratios, and density of injected electrons, control experiments, complementary transient absorption data and direct band-gap excitation of TiO2-CoPPIX films are available as supporting information.

AUTHOR INFORMATION Corresponding Author* E-mail: [email protected] ORCID Maria Abrahamsson 0000-0002-6931-1128

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The Swedish Research Council (VR) is acknowledged for financial support.

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